The present invention relates to ionography imaging methods and apparatus. More particularly, the invention relates to improvements in ionography imaging techniques which can be carried out by resorting to chambers of the type wherein an elastic dielectric receptor sheet or an analogous insulating charge-receiving medium is placed into an interelectrode gap which is defined by an anode and a cathode and contains a high Z gas. Still more particularly, the invention relates to improvements in ionography imaging chambers of the type wherein the electrodes which define the gap are portions of concentric spheres centered at the X-ray source, and to improvements in a method of making X-ray images by resorting to such chambers.
In imaging systems of the above outlined character, the high Z gas is maintained at an elevated pressure. The gas absorbs X-rays and effects the generation of a charge by a quantum process, such as the photoelectric or Compton effect. The primary and secondary electrons travel between the electrodes along field lines toward one side of the dielectric receptor sheet while the other side of the sheet abuts against one of the electrodes. The electrons produce a latent electrostatic image which is made visible by an electrostatic technique including the deposition of toner particles or in any other suitable way.
A method which can be practiced by resorting to spherical electrodes is disclosed in U.S. Pat. No. 3,803,411 granted Apr. 9, 1974 to Karl-Hans Reiss. The high Z gas (e.g., iodine-methane or a noble gas, such as Xenon or Krypton) is maintained at a pressure which exceeds atmospheric pressure, for example, at a pressure of at least six atmospheres. Since the object (especially a patient) must be protected from exposure to excessive doses of X-rays, the pressure of high Z gas (which absorbs X-rays) is preferably as high as possible. However, the pressure of high Z gas cannot be increased at will primarily for technical reasons and especially if the interelectrode gap must be accessible for removal of the dielectric receptor sheet after each exposure. Therefore, it is desirable to employ a relatively wide interelectrode gap (the width of the gap also influences the magnitude of X-ray charge to which the object must be exposed in order to obtain a satisfactory latent image). As a rule, the width of the gap is not less than 8-10 millimeters; this insures the achievement of a satisfactory yield.
However, if the width of the gap is not less than 8-10 millimeters, the latent image which is obtained by resorting to known ionography imaging chambers is unsharp, especially in the absence of correspondence or alinement between the electric field lines in the gap and the paths of X-rays from the source to the imaging chamber, i.e., if the electrodes which define the gap are not portions of concentric spheres which are centered at the X-ray source. Presently known ionography imaging chambers which employ spherical electrodes exhibit several serious drawbacks, especially in connection with the insertion and removal of dielectric receptor sheets. As a rule, the sheets are inserted by hand which is a tedious and time-consuming procedure. The dielectric receptor sheet in the gap between the electrodes must be deformed so as to follow the curvature of one of the electrodes. Such sheet is normally subjected to elastic deformation; therefore, it is preferably an extremely thin and highly elastic foil which can readily undergo elastic deformation to a degree that is needed to convert a flat sheet into a concavo-convex body. Nevertheless, only the circular central portion of the inserted and deformed sheet receives a latent image which is substantially free of distortion. Distortion of the image increases in a direction from the common center toward the edges of the electrodes and is invariably very pronounced if the receptor is a polygonal sheet, normally a square or rectangular foil. Pronounced distortion of latent images along the edges and especially at the corners of a rectangular or square sheet is attributed to lack of uniformity of distribution of stresses along the edges; the non-uniformly distributed stresses are propagated toward the common center of the electrodes when the sheet is inserted into the gap and is deformed to follow the curvature of one of the electrodes. Upon removal of the sheet from the interelectrode gap, the stresses diasppear but the latent image is distorted all the way around the center and especially at the corners.